Deduplicate handles in attach_user_blocks to reduce API calls

Add handle deduplication at the start of attach_user_blocks() to prevent
redundant API calls when the same user appears multiple times in a thread.
This helps reduce the spike in block list API calls.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[e-mail]>

tools/blocks.py

···
     from letta_client import Letta
     
     logger = logging.getLogger(__name__)
+
+    handles = list(set(handles))
     
     try:
         client = Letta(token=os.environ["LETTA_API_KEY"])